Computer assisted orthopaedic surgery system for ligament graft reconstruction

ABSTRACT

A computer assisted orthopedic surgery system for ligament graft reconstruction includes a system for obtaining data indicative of a location for ligament graft placement with respect to at least a first bone and a second bone. The system includes a position determining device that is capable of tracking the relative movements of the first and second bones using reference bodies that are attached to the first and second bones and a pointer that has a tip for contacting a surface of at least one of the first and second bones to capture one or more reference points. The system further includes a computer that is configured to determine and track intraoperative positions of the reference bodies and the pointer and to provide isometric and impingement data for a ligament graft placement based on a realistic simulation of a trajectory of a deformable ligament graft.

CROSS REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. patent applicationSer. No. 60/634,358, filed Dec. 8, 2004, which is hereby incorporated byreference in its entirety.

TECHNICAL FIELD

The present invention relates to a method and system for computerizedligament reconstruction surgery.

BACKGROUND

Injury to the anterior cruciate ligament (ACL) is the most commonligament injury in the knee, resulting in approximately 50,000reconstructions per year in the United States (Frank C B, Jackson D W,The Science of reconstruction of the anterior cruciate ligament. J BoneJoint Surg. Am 79:1556, 1997). Unfortunately, approximately 40% of ACLligaments are improperly misplaced in surgery, which can result inpost-operative knee instability, abnormal kinematics, and prematuredegeneration of structures.

Misplacements in knee surgery are common because ligament reconstructionis a technically challenging procedure for the surgeon. The knee jointcomplex is interconnected with several structural ligaments andsurrounding tissues, which may or may not be functioning normally at thetime of surgery.

Trauma related injuries which cause ACL rupture often entail damage toother structures surrounding the knee joint, such as the medial or thelateral collateral ligaments. Additionally, posterior cruciate ligament(PCL) ruptures or tears can occur and require diagnosis and/or surgicalreconstruction. These surrounding structures are also critical forinsuring adequate knee function, rotatory stability, and normalkinematics.

Although various clinical tests for assessing knee instability exist, itis extremely difficult for the surgeon to objectively take these resultsinto consideration when determining the optimal course of a ligamentgraft. In particular, the magnitudes and directions of the laxities andtheir interrelationships to the underlying anatomical structures can beelusive, with the combined translational and rotational motions of theknee joint. Moreover, these interrelationships may change over thecourse of the surgery as structures are removed and/or reconstructed.

One method for determining the femoral point of graft attachment duringACL replacement is disclosed in European Patent Application No. 0603089to Cinquin et al., which is hereby incorporated by reference in itsentirety. The disclosed method concerns the determination of a femoralpoint of graft attachment with respect to a tibial graft attachmentpoint such that the distance between these two points remains invariantduring knee flexion and extension. The positions within an on-sitethree-dimensional coordinate system of a reference and a pointer, whichare both provided with energy emitting markers, are determined by meansof a three-dimensional position measurement system, such as the OPTOTRAKposition measurement system, Northern Digital, Waterloo, On. Theposition measurement system measures the position of the markers withrespect to the on-site three-dimensional coordinate system. Therewith,the position of the tip of the pointer is determinable by means of acomputer.

The Cinquin method includes the steps of (1) attachment of a firstreference at the tibia; (2) positioning of the pointer tip at apreviously determined point T1 and measuring the position of the pointertip with respect to the first reference; (3) positioning of the pointertip at several points P1 at the trochlea of the femur close to thatposition where the invariant point is expected; (4) calculation of thedistances of point T1 and each of the points P1; (5) displacement of thefemur with respect to the tibia and calculation of the variations of thedistances between T1 and each of the points P1; and (6) selection ofthat point P1 among points P1 which shows the most invariant distance.

The Cinquin method assumes that the optimal placement of a graft isdetermined by a simple isometric elongation criterion which relies onthe ligament trajectory following the course of a straight line (i.e.the trajectory is not dependent on the intrusion of the bone surfacesnor the ligament thickness).

Ligament grafts in reality are complex structures that have anappreciable thickness (e.g., 8-10 mm in diameter), which can affecttheir function and geometry during joint motion. In particular, thecourse of these ligament bundles is often guided by the curvedprotruding surfaces on the ends of the femur and on the tibia in thevicinity of the ligament attachment sites. Indeed, the collateral andposterior structures of the knee are known to wrap around the curvedbone surfaces of the femur and tibia which can influence the ligamenttrajectory and elongation patterns during knee motion. As a result, alinear based determination of the isometric elongation characteristicsof the ligament is too simplistic and does not account for the realitiesof a typical operative site where the ligament does not follow a purelylinear path.

Another disadvantage of the Cinquin method is that only a normal flexionextension motion of the knee joint kinematics is considered in thedetermination of the fixation point of the ligament graft, and noconsideration is taken for the surrounding structures and theirassociated laxities.

U.S. Pat. No. 6,725,082 (which is hereby incorporated by reference inits entirety) discloses an image-based system and method forcomputer-assisted ACL replacement in which landmark points areidentified on the bones and on medical images of the knee (such as Xrays, CT or MRI scans), the images are then registered to the patient,and a drilling tool is then navigated with respect to some “anatomical”graft placement criterion as determined on the medical images. Thispermits visualisation of the graft fixation points on the bones in theX-ray images, and in relation to the landmark points identified on theimages. The disadvantage of this system is that no information ormeasurements regarding knee motion or laxity is provided. Furthermore,no means are provided for simulating realistic ligament trajectoriesbased on the 3D shapes of the bones and of the ligament geometry such asthe graft thickness and length.

Yet another disadvantage of conventional computer positioning systems,such as those described above, is that these systems are based almostentirely on determining optimal isometric coordinate points or locationsfor determining a fixation point on the femur and a fixation point onthe tibia. However, there are a number of other considerations thatshould or can be taken into account when determining the optimallocation of the ligament and for assisting the physician in selectingthe appropriate procedure that is to be performed on the patient. Forexample, knee laxity data can assist the physician in determining thetype of procedure to be undertaken; however, this type of data is notutilized in the conventional computer systems, which again, merelyutilize isometric data to model a straight line (linear) layout for theligament.

SUMMARY

The present invention relates to a system for reconstructing ligamentsthat connect to at least two bones. The invention aids the surgeon indetermining the course of a replacement ligament by taking intoconsideration the laxities of the involved joint, as well as thegeometric properties of the ligament graft, and the impingement of thebones onto the graft.

In one aspect of the invention, a computer-assisted orthopedic surgery(CAOS) system is provided and is configured for performing ligamentreconstruction procedures on a patient, such as those performed in kneesurgery. The system includes a position measurement device incommunication with a computer to determine the position and orientationof objects in a three dimensional coordinate system. The threedimensional coordinate system includes at least one bone, such as thepatient's femur or tibia. Objects to be tracked comprise at least threemarkers (or triplets), which can be configured to emit, receive, orreflect energy, such as light or acoustic energy.

To sense the position of light reflecting markers, the system includesat least two detecting elements, such as two cameras. The two camerasdetect the light reflected from the light reflecting markers todetermine the position of each marker associated with an object to betracked. Based on the respective positions of markers associated withthe tracked object, the position and orientation of the tracked objectare determined.

The system preferably includes a plurality of reference bodies that canbe used to determine the position and orientation of a patient's bone.The reference bodies can be rigid members having at least three markerseach. Each reference body preferably includes an attachment element,such as a screw or pin, with which the reference bodies can be attachedto a bone. For example, respective reference bodies can be attached tothe femur and tibia.

The system also can include a pointer. The pointer includes markers thatallow the position and orientation of the pointer to be determined. Thesystem also includes a calibration device that can be used to measurethe relationship between the pointer tip and the markers. Thus, theposition of the pointer tip can be determined from the positions of themarkers relative to the three-dimensional coordinate system.

The computer can be configured to determine the position and orientationof the reference bodies and pointer based upon the position andorientation of the associated markers.

Moreover, the system can be configured for identifying and applying ananatomical coordinate system to at least two bones of the joint. Theanatomical coordinate system can include directions such asmedial-lateral, proximal-distal, anterior-posterior, and so on.

In a further aspect, the present invention can provide a system fordetermining intra-operatively the three dimensional shape of the bonesurface in the vicinity of the articulating joint and in particular inthe vicinity of the ligament graft fixation points associated with thejoint. In particular, the three dimensional shapes of the involved bonesmay be provided with image free techniques.

In another aspect, the present invention can provide a system forpassively guiding the manipulation of a first bone relative to a secondbone, in relation to a predetermined anatomical coordinate system. Inparticular, these passive motions include translational and rotationaldisplacements in directions “abnormal” to the primary functional motionof the joint (e.g. axial rotation of the knee), in addition to “normal”rotations about the primary functional axes of the joint (e.g. kneeflexion). Examples of such kinematic protocols can be found in Scuderi GR. Scott N W, Classification of Knee Ligament Injuries In Insall Scott(Eds.) Surgery of the Knee, Chapter 29, pp. 585-599. In addition, jointangles can be determined and displayed in real time during the passivemanipulations (e.g. flexion angle=20°).

In still another aspect, the present invention can provide a system formeasuring the relative motion of one bone with respect to another bone,and for projecting the measured trajectories and amplitudes of motion ofvarious points onto various planes and directions in relation to apredetermined anatomical coordinate system. For example, to evaluatetranslational or rotational stability the trajectories of the medial andlateral tibial plateau points can be measured and displayed in relationto the femur during the kinematic acquisition. The maximum amplitude ofdisplacement with respect to the femur of each point can then becalculated. These results can then be summarized as displacementmagnitudes, or as components in a particular direction of the anatomicalcoordinate system, such as in the anterior-posterior direction.

And in yet another aspect, the present invention can provide a systemthat displays numerically and graphically on a screen the patterns, theamplitudes, and the projections of the measured trajectories.

Also in another aspect, the present invention can provide a system thatdisplays graphically in real time a deformable and bendable model of theligament graft superimposed on three dimensional representations of theinvolved bones.

In yet another aspect, the present invention can provide a method fordisplaying graphically isometric and impingement data superimposed onthree dimensional representations of the involved bones.

Still further, the present invention can provide a system for displayingin real time the position and orientation of a surgical drilling tool inrelation to the targeted graft tunnel position and orientation.

According to one embodiment, a computer assisted orthopedic surgerysystem for ligament graft reconstruction includes a system for obtainingdata indicative of a location for ligament graft placement with respectto at least a first bone and a second bone. The system includes aposition determining device that is capable of tracking the relativemovements of the first and second bones using reference bodies that areattached to the first and second bones and a pointer that has a tip forcontacting a surface of at least one of the first and second bones tocapture one or more reference points. The system further includes acomputer that is configured to determine and track intraoperativepositions of the reference bodies and the pointer and to provideisometric and impingement data for a ligament graft placement based on arealistic simulation of a trajectory of a deformable ligament graft.

These and other features and aspects of the invention can be appreciatedfrom the accompanying figures and the following description.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

The foregoing and other features of the present invention will be morereadily apparent from the following detailed description and drawingfigures of illustrative embodiments of the invention in which:

FIG. 1 is a schematic of a computer-assisted orthopedic surgery (CAOS)system according to one embodiment;

FIG. 2 is a schematic view of a main user interface screen for inputtinginformation related to the surgery;

FIG. 3 is a schematic view of a vision screen to assist the physician inpositioning of a camera such that the desired tools are within an entireoperative field;

FIG. 4 is a schematic view of a screen showing a complete slow and fluidflexion-extension movement to assist the physician in determining theanatomy of the patient;

FIG. 5 is a schematic view showing the digitization of a plurality ofreference points that are taken for the tibia of the patient and for usein generating a morphological model of the investigated bone portion

FIG. 6 is schematic view showing the digitization of a plurality ofreference points that are taken for the femoral arch of the patient andfor use in generating a morphological model of the investigated boneportion;

FIG. 7 is a schematic showing additional digitization of referencepoints that are taken for the tibia;

FIG. 8 is a schematic view showing the digitization of a plurality ofreference points that are taken for the femur and for use in generatinga morphological model of the investigated bone;

FIG. 9 is a schematic view of a graphic representation of resultsobtained from an anterior drawer test which is part of a number ofpreoperative laxities tests that are performed to test for jointlaxitity;

FIG. 10 is a schematic view of a graphic representation of resultsobtained from a medio-lateral stability test which is part of a numberof preoperative laxities tests that are performed to test for jointlaxitity;

FIG. 11 is a schematic view of a graphic representation of resultsobtained from a Lachman test which is part of a number of preoperativelaxities tests that are performed to test for joint laxitity;

FIG. 12 is a schematic view of a graphic representation of resultsobtained from a pivot shift test which is part of a number ofpreoperative laxities tests that are performed to test for jointlaxitity;

FIG. 13 is a schematic view of a graphic representation of the resultsof the joint laxities test displayed on the single screen;

FIG. 14 is a schematic view of a graphic representation of a selectionof a tibial point and a femoral point as part of an anisometryinvestigation;

FIG. 15 is a schematic view of a graphic representation of a selectionof fixation points by navigating drill guides by aiming the plannedfixation points to select the points for analysis of the anisometricdata;

FIG. 16 is a schematic view of a graphic representation of a tibialinsertion point being digitized by the use of a pointer to select aninsertion point of a tunnel;

FIG. 17 is a schematic view of a graphic representation of a femoralinsertion point being digitized by the use of a pointer to select aninsertion point of the tunnel;

FIG. 18 is a schematic view of a graphic representation of animpingement study to estimate any impingement/contact between theligament (fiber) and a roof of a notch of the femur;

FIG. 19 is a schematic view of a graphic representation of thedigitization of the femoral surface of insertion of the medialcollateral ligament using a pointer or the like;

FIG. 20 is a schematic view of a graphic representation of thedigitization of an entrance or insertion point of the tibial tunnelusing the pointer;

FIG. 21 is a schematic view of a graphic representation to check theanisometry of the medial collateral ligament by evaluating a graph/plotand an anisometry value;

FIG. 22 is a schematic view of a graphic representation of thedigitization of the femoral surface of insertion of the external lateralligament to permit accurate bone morphing;

FIG. 23 is a schematic view of a graphic representation of thedigitization of the Gerdy's tubercle with the pointer to define a tibialinsertion point for the lateral collateral ligament in LCLreconstruction;

FIG. 24 is a schematic view of a graphic representation of thedigitization of the femur insertion point of the LCL with the pointer;

FIG. 25 is a schematic view of a graphic representation of navigation ofdrill guides by aiming the planned points;

FIG. 26 is a schematic view of a graphic representation of theanisometry of the LCL which is displayed to the physician in terms ofanisometry data and value, with the simulated bone morphs and tunnellocations being displayed;

FIG. 27 is a schematic view of the results (values) obtained during thelaxitity tests for viewing by the physician;

FIG. 28 is a schematic view of selected fixation points on the tibia andfemur used in a method according to the present invention for computing,in real time, the ligament course;

FIG. 29 is a schematic view of a virtual ligament that is calculated anddisplayed to determine if there is impingement between the virtualligament and any bone surface;

FIG. 30 is a schematic view of a shifted ligament computed to avoid anyimpingement that existed between the virtual ligament and the bonesurfaces; and

FIG. 31 is a schematic view of a computed ligament path that representsthe best fit and avoids impingement between the ligament and any bonesurface.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring now to FIG. 1, a computer-assisted orthopedic surgery (CAOS)system 10 is schematically shown. The CAOS system 10 is configured forperforming ligament reconstruction surgeries, such as knee ligamentreconstruction surgery. The system includes a suitable positionmeasuring device 20 that can accurately measure the position of markingelements in three dimensional space. The position measuring device 20can employ any type of position measuring method as may be known in theart, for example, emitter/detector or reflector systems including optic,acoustic or other wave forms, shape based recognition trackingalgorithms, or video-based, mechanical, electromagnetic and radiofrequency systems. In a preferred embodiment, schematically shown inFIG. 1, the position measuring system 20 is an optical tracking systemthat includes at least one camera that is in communication with acomputer system 30 and positioned to detect light reflected from anumber of special light reflecting markers, or discs 50.

Detecting and determining the position and orientation of an object isreferred to herein as “tracking” the object. To provide precisiontracking of objects, markers 50 can be rigidly connected together toform reference bodies, (e.g., 100, 110), and these reference bodies canbe attached to bones, tools and other objects to be tracked. One suchdevice that has been found to be suitable for performing the trackingfunction is the Polaris™ system from Northern Digital Inc., Ontario,Canada.

The position measurement device 20 is described in greater detail in anumber of publication, including U.S. Pat. Nos. 5,564,437 and 6,725,082,both of which were previously incorporated by reference.

The position of the patient's bones, such as the patient's femur 2 andthe patient's tibia 4, can be determined and tracked by attachingreference bodies 100, 110, which include respective markers 102, 112.Reference bodies can be attached to bones or tools using pins or screws(104, 114), or various quick release mechanisms. The reference bodiescan also be shaped in the form numbers (e.g., “1”, “2”, “3” . . . ) oralphabetical letters, such as “F” for Femur, “T” for Tibia, “P” forpointer, and so on, so as to avoid confusion as to which reference bodyshould be attached to which bone or tool.

The tracked objects and there relative positions can be displayed on ascreen that is connected to the computer system 30. In a preferredembodiment, the display is a touch screen which can also be used fordata entry.

The position measurement device 20 includes a number of different toolsthat are used at different locations and perform different functions asthe system 10 is operated to yield optimal ligament graft reconstructiondata and information. These tools include the above described markers50, which act as landmark markers, as well as other tools, such as adrilling device 70 having at least three markers 72 is an example of anobject trackable by position measuring device 20. The system alsoincludes a pointer 120, with markers 122, which can be used to digitizepoints on the surfaces of the femur 2 and tibia 4.

The drilling device 70 also has a drill tip 74 having a known spatialrelationship relative to markers 72. Position measuring device 20determines the position and orientation of markers 72 in the threedimensional coordinate system 40. Based upon the known spatialrelationship between drill tip 74 and markers 72, the position of drilltip 74 is determined.

Computer 30 is preferably configured to allow at least one of medicalimage data and ultrasound data to be used in planning the position andorientation (path) of a hole to be drilled in a bone. The path of a holebeing bored by the drill 70 can be monitored and displayed by thecomputer 30. Thus, the actual path can be compared to the previouslyplanned drill path to allow the practitioner to minimize deviationsbetween the actual procedure and the preoperative plan. In oneembodiment the drill 70 is guided to allow the computer 30 to controlthe drilling path.

The ligament reconstruction system 10 also includes a plurality ofreference bodies 100, 110, for determining the position and orientationof an individual's bone in the three dimensional coordinate system 40.The reference bodies 100, 110 are preferably rigid and includerespective markers 102, 112, which are preferably configured to emitenergy. Each reference body 100, 110 preferably includes a respectiveattachment element, such as pins or screws 104, 114, with which thereference bodies can be releasably attached to a bone. For example, thereference body 100 is shown as being attached to femur 2. The positionand orientation of femur 2 can be determined based upon the position andorientation of markers 102 attached thereto. Markers 102, 112 aresufficient to establish the position and orientation of the rigid bodies100, 110 within the coordinate system 40.

The system 20 also includes a pointer 120 and endoscope 130, whichcooperate to allow a practitioner to digitize landmarks of the femur 2and tibia 4. Digitizing a landmark comprises determining the position ofthe landmark in the three dimensional coordinate system, as discussedbelow. The pointer 120 includes markers 122, which allow the positionand orientation of the pointer 120 to be determined in the threedimensional coordinate system 40. The pointer 120 preferably includes apointer tip 124 having a known spatial relationship to the markers 122.Based upon the known spatial relationship, the position of the pointertip 124 can be determined from the position and orientation of themarkers 122.

In a preferred embodiment of the invention landmark points and ordirections are digitised with respect to the femur 2 and tibia 4 withthe pointer and are stored in the computer. Preferably an anatomicalcoordinate system for the femur and the tibia is defined based on atleast a portion of the acquired data. The coordinate system could alsobe defined at least partially using kinematic methods, such as fitting aplane to a trajectory (e.g. fitting the sagittal plane to theflexion/extension trajectory). Possible landmark points include but arenot limited to the tibial plateau glenoids, spine, malleoli, the femoralnotch, condyles, hip center, bone surface areas, etc.

The ligament reconstruction system 10 of the present invention ispreferably an integrated system in which each of the tools is incommunication with a master controller, such as the computer 30, whichserves to collect all of the data from the individual tools and thenprocess the data to calculate various measurements that are displayableto the physician. The ligament reconstruction system 10 accordinglyincludes a user interface 200 that is supported by software in thecomputer 30. The user interface 200 is configured to assist and walk thephysician through the ligament reconstruction procedure to obtainoptimal results and to assist the physician in determining what the bestcourse of action is in terms of providing and optimizing the stabilityof the knee.

FIG. 2 illustrates a main user interface page or screen 210 that ispreferably displayed on a display 32 (FIG. 1) of the computer 30. Themain user interface screen 210 includes a section 212 where thepatient's name or other identifying information can be entered using akeypad or the like 214. The main screen 210 also includes a kneeindicator box or region 216 which the physician or operator canhighlight whether the procedure is being performed on either the leftknee (box 217) or the right knee (box 218). The main screen 210 includesa region 220 where tool related information can be entered, as byhighlighting a single guide box 221 or a “2 guide” box 222, etc. Themain screen 210 further includes a section or region 224 where differentoperations to be performed during the procedure can be selected by thephysician. For example, the region 224 can list a number of differenttests that are performed to test the condition and strength of the knee,especially the ligaments thereof, to assist in diagnosing a kneecondition, such as a torn ligament (e.g., ACL) or decreased stability inthe knee.

In one embodiment, the region 224 lists a number of tests that areassociated with the laxity of the joint, in this case the knee. Laxityof a knee is generally the degree of looseness or literally the laxnessof the knee and can be determined and tested by moving the tibia 4relative to the femur 2 and measuring the relative movement of the tibia4 relative to the femur 2. In the illustrated embodiment, the region 224includes 4 boxes or sub-regions that can be highlighted and chosen bythe physician to test and evaluate knee laxity. The region 224 includesa first box 226 that represents the first test (the Lachman Test); asecond box 228 that represents the second test (Anterior Drawer Test); athird box 230 that represents the third test (Pivot shift Test); and afourth box 232 that represents the fourth test (Medial-Lateral StabilityTest).

Other helpful indicia are indicated and provided on the main screen 210to assist the physician during the operative procedure. For example, themain screen 210 can include icons that indicate whether certain toolsare operative and in communication with the computer 30. Morespecifically, these icons can include a first icon 240 that includes ahighlightable letter “P” that represents the pointer 120; a second icon242 that includes a highlightable letter “T” that represent the secondreference body 110 that is for attachment to the tibia 4; and a thirdicon 244 that includes a highlightable letter “F” that represents thefirst reference body 100 that is for attachment to the femur 2. Asdescribed below, the software and operating system is configured so thatwhen any one of the tools (pointer 120 and reference bodies 100, 110)are in the field of view (vision) for the detection system (e.g.,camera), which is necessary for the proper transmission of referencedata to the computer 30, the respective icon 240, 242, 244 ishighlighted. This makes it easy for the physician to glance at the mainscreen 210 and see whether all of the tools are in the proper line ofsight with the positioning measuring device 20, thereby permitting thefree transmission of data (location data) from the tools to the computer30.

More specifically and as shown in FIG. 3, a vision screen 250 isillustrated and is used to assist the physician in positioning of acamera, that is associated with the position measurement device 20, suchthat the desired tools are within the entire operative field. In theillustrated embodiment, the first and second reference bodies 100, 110are within the field of vision as evidenced by the P icon 252 and the Ficon 254 that are visible in the delineated field of vision 256 whichrepresents the entire operative field. The icons 242, 244 are likewisehighlighted (as being illuminated in a different color) to indicate thatthe reference bodies 110, 100 are visible within the operative field asit is presently set. It will be appreciated that the operative field andthe field of vision 256 depends upon a number of parameters, includingthe precise location of the patient on the operating bed as well as thelocation of the camera. If either of these parameters changes, then thefield of vision 256 will likewise change and one or more of the toolsmay not be visible to the position measurement device 20. For example,if the patient unexpectedly moved on the operating bed, the patient'sleg could move such that the reference bodies 100, 110, attached to thefemur 2 and the tibia 4 could fall outside of the field of vision 256.If this occurs, one or more of the icons 252, 254 would move outside ofthe field of vision 256 on the screen 250 and the physician would thentake the appropriate remedial action, such as moving the patient's leguntil both icons 252, 254 lie again within the field of vision 256. Theillustrated field of vision 256 includes both a diameter display 257 aswell as a depth display 258 (which is a two dimensional representationof the field of vision) and it is important that the tools lie withinboth the field of vision in both displays 257, 258.

Before the pointer 120 is used to determine any position taken by itstip 124, the positioning measuring device 20 is preferably calibratedusing any number of different techniques. For example, the pointer 120is calibrated by placing the pointer tip 124 in a cone of a calibrationblock. It is important that the pointer should not move during thecalibration process. The icon 240 (FIG. 2) is highlighted indicated thepointer 120 is in the field of vision. During the calibration of thepointer 120, the pointer 120 can be slightly tilted a predeterminedangle (e.g., 30 degrees) while keeping the tip 124 within thecalibration cone. The pointer 120 fitted with markers 122 interacts withthe detection system so as to precisely determine any position taken byits tip 124, thereby ensuring calibration. Similarly, a drill guide 78of the drill 70 is calibrated by selecting precisely the externaldiameter of the tube of the drill guide 78 and then the guide 78 iscalibrated keeping its axis in the groove of a calibration block. Thedrill guide 78 is turned or rotated by a predetermined number of degrees(e.g., 30 degrees) around its axis, keeping it in the groove in order toensure the accuracy of the calibration. The extremity of the drill guide78 is placed on the calibration plane perpendicular to the surface. Thephysician then operates an actuator, such as a foot pedal, to initiatecalibration of the drill guide 78.

To perform computer assisted orthopedic surgery, especially ligamentgraft reconstruction, using the present system 10, the reference body100 is attached to the femur 2 at a selected point, as by screwing thereference body 100 into the bone with screw 104. The reference body 100includes the first set or triplet of photoemitters 102 that are thusfixed to the femur 2 at the point. Similarly, the reference body 110 isattached to the tibia 4 at a selected point by screwing the body 110into the bone with screw 114. The reference body 110 includes the secondset or triplet of photoemitters 112 that are likewise fixed to the tibia4. Since the reference bodies 100, 110 are fixed to the bones 2, 4, thepointer tip 124 can be used to point at any given point on bone 2, 4,the position of which is precisely taken by its tip 124. Then, it ispossible using a conventional data processing system to determine thevector connecting the point where the respective body 100, 110 attachesto the bone 2, 4 to the point of the tip 124, and therefore, locate anygiven point for any position of the femur 2 or tibia 4.

According to one exemplary method, a number of different referencepoints are obtained using a digitization technique that is part of abone morphing system. For example, the pointer tip 124 is used todigitize the external malleolus by placing the pointer tip 124 at thisbone location which is determined and stored by position measurementsystem 20. As can be seen, the icons 240, 242 are highlighted indicatingthey are in the field of vision. Next the internal malleolus isdigitized by placing the pointer tip 124 at this location. The leg isplaced in complete extension and neutral rotation as shown in FIG. 4 andthis position is used to project the femur 2 onto the tibia 4 duringnavigation (icons 242, 244 highlighted). A complete slow and fluidflexion-extension movement is performed as shown in FIG. 4. A posteriordrawer is forced in order to reduce laxities. Since the reference bodies100, 110 (not shown) remain fixed to the femur 2 and the tibia 4, therelative movements can be recorded.

According to the present invention, additional reference points aregathered. More specifically, another reference point that is determinedand mapped is the summit of the external spline 266 of the tibia 4 whichis digitized using pointer 124 as shown in FIG. 5. Once again, thislocation is easily determined using a conventional data processingsystem since a vector can be generated between the summit point and theanchor point of the reference body 110 and therefore, the externalspline summit point can be determined. Similarly, the internal splinesummit 267 is digitized using pointer 120 as shown in FIG. 5.

Additional reference points are taken for the tibia 4 by placing thepointer 120 on an external glenoid 268 as shown in FIG. 5 to therebydigitize the external glenoid 268 (icons 240, 242). An internal glenoid269 is digitized using the pointer tip 124 as well as a middle of aninterior inter-meniscal ligament 270 of the tibia 4. The glenoids 268,269 represent tibia plateau surfaces. Thus, the precise locations of thetwo glenoids 268, 269 and the middle of the anterior inter-meniscalligament 270 are calculated using the position measurement system 20 andas will be described hereinafter, these locations or points on theidentified morphology of the patient's bones are used in the presentsystem to calculate the joint (knee) laxity to assist in an improvedrestorative knee operation. During this digitization process, theoperator merely runs the pointer tip 124 along the bone and then when itis desired to collect a reference point, the operator simply manipulatesthe system as by pressing a button or the like to signal the system tocollect data for the selected reference point.

Acquisition or data points are also taken for the femur bone 2 and moreparticularly, the pointer 120 is moved along the surface of a femoralnotch 5 of the femur bone 2 in order to digitize the middle of the archof the femoral notch 5 at approximately, the “12 o'clock position”, inpractice in the prolongation of the lateral border of the posteriorcruciate as shown in FIG. 6. Next additional selected surfaces of thetibia 4 and the femur 2 are investigated and data points are taken andmore particularly, the peri and inter-spinal surface of the tibia 4 isdigitized using the pointer 120 as shown in FIG. 7. In all of thesesteps, the digitization occurs by locating and logging a selected bonesurface point in the three dimensional coordinate system 40 and by usingthe data processing system. In other words, the pointer 124 is movedalong the bone surface and coordinate data are gathered for use ingenerating a morphological model of that bone portion.

Yet another feature of the present invention is shown in FIG. 7 in whicha bar meter 280 or the like is shown for indicating the amount of datapoints that have been captured and digitized and stored as acquisitionpoints. In other words, the bar meter 280 is initially empty indicatingthat no data points have been captured and as the physician moves thepointer 120 around the femur bone surface (peri and inter-spinalsurface), a bar 282 begins to grow within the meter 280. As moreacquisition points are captured, the length of the bar 282 grows and theuser is generally made aware of the amount of acquisition points thatleft before the system 10 deems the data acquisition step complete andmoves on to the next step or operation (e.g., next data acquisition).Accordingly, once the bar 282 extends completely to the right of themeter 280 and fills up the entire meter box 280, then the specific dataacquisition step is deemed to be complete since a sufficient number ofdata points have been acquired. The meter 280 helps assist, in realtime, the progress of the data acquisition and for example, if the userhas concentrated too heavily on collecting data points in one particularlocal area, while ignoring other important areas, then the everincreasing length of the bar 282 in the meter 280 can serve as areminder that it is time to move on to the other areas to collect data.

As previously mentioned, the pointer 120 can be actuated to begin dataacquisition by simply activating an actuator, such as a foot pedal inwhich case it is merely depressed by the physician. The above describeddigitizing process can be thought of as a landmark acquisition process.Thus, according to a preferred embodiment of the present invention,landmark points and/or directions are digitized with respect to thefemur 2 and the tibia 4 utilizing the pointer 120 and are stored in thecomputer 30. Preferably, an anatomical coordinate system for the femur 2and the tibia 4 is defined based on at least a portion of the acquireddata. The coordinate system can also be defined at least partially usingkinematic methods, such as fitting a plane to a trajectory (e.g.,fitting the sagittal plane to the flexion/extension trajectory). Asdiscussed in some detail above, possible landmark points include but arenot limited to the tibial plateau glenoids, spine, malleoli, the femoralnotch, condyles, hip center, bone surface areas, etc. The dataacquisition points can be used to determine other information, such aslocating a mechanical axis of the tibia 4.

In a preferred embodiment of the present invention, three dimensionalgeometrical surface models of the bones are provided by image-freemeans. Preferably these models are obtained by adjusting a deformablemodel of the bone to points acquired on the bone surface. Examples ofsome known methods of carrying out this task can be found in thefollowing references: (1) “Building a complete surface model from sparsedata using statistical shape models: application to computer assistedknee surgery” by M. Fleute and S. Lavallee, published in Medical ImageComputing And Computer-Assisted Intervention—MICCAI'98, Spinger-VerlagLNCS Series, pages 880-887, October 1998; (2) Fleute M, Lavallee S,Julliard R. Incorporating a statistically based shape model into asystem for computer-assisted anterior cruciate ligament surgery. MedicalImage Analysis. September 1999; 3(3):209-22. However, other knownmethods of obtaining geometrical bone models in surgery exist (forexample, matching medical image data such as CT, MRI, etc, to pointsacquired with a pointer or an ultrasound probe). Each of the abovelisted references is hereby incorporated by reference in its entirety.

In particular, the three dimensional shapes of the involved bones may beprovided with image free techniques, such as using bone morphingsoftware which is capable of extrapolating very few range data to obtaina complete surface representation of an anatomical structure (e.g., abone). The specific details of bone morphing are set forth in the abovereferences but in general, a complete surface model is built from sparsedata using statistical shape models. In particular, the model is builtfrom a population of a number of specimen (points), such as femur ortibia points, that are digitized. Data sets are registered togetherusing an elastic registration method (e.g., the Szeliski and Lavalleemethod) based on octree-splines. Principal component analysis (PCA) isperformed on a field of surface deformation vectors. Fitting thisstatistical model to a few points in performed by non-linearoptimization. Results can thus be presented for both simulated and realdata. This method is very flexible and can be applied to any structuresfor which the shape is stable.

In a preferred embodiment of the invention representations of the femurand tibial bones, or a portion thereof, are displayed on the screen.These models move on the screen in real time based on the motion oftracked femur and tibia. In a preferred embodiment of the invention thesystem guides the surgeon in manipulating the knee, by using the bonerepresentations as visual aids. Knee manipulations are preferablypreformed for both “normal” and “abnormal” joint motions. Normal motionscan include passive flexion/extensions of the knee, with the tibiaguided by the medial and lateral femoral condyles. Abnormal jointmotions are those indicative of instability in the joint, such as thosedescribed in abovementioned chapter by Scuderi.

In other words, the present system preferably is configured to collectdata with the pointer 120 in order to perform bone morphing operationsor procedures to thus obtain three-dimensional geometrical surfacemodels of the bones. During typical bone morphing, the representation ofthe bone (in this case the tibia 4) on the display 32 of the computer 30includes shading or some other type of indicator that highlights theareas of the bone that have increased accuracy due to the physicianhaving collected a number of data acquisition points in this region. Forexample, the areas where more data points were collected are shadeddifferently or displayed in a different color and the accuracy of thebone morphing (i.e., tibial bone morphing) on the cartilaginous and bonysurface can be checked. Areas of the bone that have not been scannedusing the pointer 120 are simulated using conventional extrapolationtechniques as commonly found in bone morphing techniques and systems.

Similarly, target locations of the femur 2 are digitized and asillustrated in FIG. 8, and in particular, a roof and the lateral notchof the femur 2 are preferably digitized as well as the anterior arch inthe area of potential conflict as shown in FIG. 8 (note: icons 240, 242are highlighted and bar 282 in the meter 280 indicates the number ofdata acquisition points collected). On the display screen, the systempreferably indicates where the pointer 120 has travelled and collecteddata acquisition points as by providing a visual colored pointrepresentation on the screen. As with the tibia 4, bone morphing for thefemur 2 is performed and a representative bone morph is displayed to thephysician. Shading or some other type of indicator is used to highlightthe areas of the bone that have increased accuracy due to the physicianhaving collected a number of data acquisition points in this region (inthis case around the arch and notch).

In order to assess the patient's condition and to better assist thephysician in diagnosing the patient's condition and recommending aremedial action, the present system 10 preferably guides the physicianthrough a number of different motion protocols that are part of tests tobe performed to assess the patient's condition. For example, a number ofpreoperative laxities tests can be formed to test for joint laxity. Onesuch test, shown in FIG. 9, is an anterior drawer test and involvesmanipulating the bones (femur 2 and tibia 4) through a range ofmovements and collecting data as a result of the reference bodies 100,110 being attached thereto. More precisely, the test is performed withthe patient lying on their back with their knee in 90 degrees offlexion, with the foot resting firmly on the table. The physician graspsthe top portion of the skin with both hands, positioning the thumbs oneither tibial condyle 13, 15. Stabilizing the foot, the physician placespressure slowly on the proximal tibia by moving the shin toward thephysician. Abnormal looseness and movement forward indicated asignificant ACL injury. In other words, the motion and track of thereference bodies can easily be observed and stored in memory and fromthis data, the looseness and movement can be quantified and displayed tothe physician in real time on the display screen.

In one aspect of the present invention, an on-screen guide 17 isprovided to summarize measured test values for the data acquisitionpoints for the drawer test and offer a unique visual tool to thephysician to show in real time how the current position between thefemur 2 and tibia 4 corresponds to other positions between the twobones. More specifically, FIG. 9 shows a user interface tool 17 whichdisplays the movement of the tibia 4 relative to the femur 2 during thelaxity test, in this case the drawer test. In particular, the tool 17has a first indicator 19 to represent the results of the medialstability measurement, a second indicator 21 to represent the results ofthe lateral stability measurement, and a third indicator 23 to representthe results of the axial rotation measurement. Each of the indicators19, 21, 23 graphically illustrates a range of the data for therespective measurement (medial and lateral stability and axial rotation)taken for the captured reference points and each includes a locator 25that highlights the measurement value for the current position of thetibia 4 relative to the femur 2.

In the illustrated embodiment, the locator 25 is in the form of a ring,of different color, that surrounds the respective indicator which is inthe form of a column. The locator 25 also shows the exact measurementvalue next to the column. The relative position of the locator 25axially along the column represents how the current measurement valuecompares to the other measurement values. For example, FIG. 9 show thatthe current position between the tibia 4 and femur 2 results in a medialstability value of 3 mm (anterior) (as indicated by the highlighted “3”next to the bar column) which lies closer to the maximum measured valueof 5 mm (anterior) and far away from the posterior measured value of 4mm. The range of the bar column is 9 mm (anterior 5 mm+posterior 4 mm).The other indicators displays (columns) 21, 23 have similar locators 25so that the operator, in real time, can simply look at the screendisplay and see how the acquired values for the current bone positioncompares to other bone positions. In another aspect, the neutralposition along the indicator (column) can be indicated as a line orinterface between two different colored column portions. For example,the column (indicator) has two different colors along its axial length,with the neutral position being indicated where the two differentcolored portions abut. This demarcation permits the operator to easilyascertain the neutral position and see in real time how far away orclose the current position is from the neutral position and how movementof the tibia 4 changes this relative position.

It will be understood that the guide tool 17 (indicators) can be used toshow the measurements of the other laxity tests and indicate the neutralpositions of the test measurements.

A second pre-operative test is a medio-lateral stability test shown inFIG. 10, while a third test is a Lachman test shown in FIG. 11. In theLachman test, the femur 2 is grasped with one hand, while the tibia 4 ispulled forward and the amount of excursion noted. In normal subjects, noforward movement is elicited.

A fourth test that tests for laxity is the pivot shift test which isshown in FIG. 12. The location of the anterior inter-meniscal ligament(represented by previously acquired points) is used during the pivotshift test and more specifically, it is this point that is the preferredreference point of the femur 2. The pivot shift test is a test foranterior cruciate stability. When this ligament is lax, the pivotbetween the lateral femoral condyle 13 and the tibial condyle 15 isunstable. Thus, if the knee is medially rotated, the tibia can bedisplaced forwards with respect to the femur 2. This subluxation isspontaneously reduced when the knee is flexed, giving a palpable orvisible jerk. One of the disadvantages of conventional pivot shifttesting was that there was no associated value or quantification of theresults; but rather, the physician would simply manipulate the leg andthen use his past experiences to assess the patient's current condition.

To generate a value representing the result of the pivot shift test, twoplots can be plotted and compared to one another, namely a first plot(reference or neutral plot) that represents the laxity as a function ofdegree of flexion of the patient's leg prior to performing the pivotshift and a second plot which plots the motion of the leg after thepivot shift (dislocation) has been performed. As would be expected, thelaxity value over the degree of flexion is greater for the plot afterthe pivot shift is performed on the knee and therefore, there is adistance between the two plots. Thus, for any given angle of flexion,two values can be compared, one value for the neutral knee position andanother value for the knee with the pivot shift, thereby permitting thedifference between these two values to be quantified.

Preferably, the system and software is configured to look for anddetermine the maximum distance or maximum translation between theneutral and pivot shift positions for each given angle of flexion andthis value shown in FIG. 13 is displayed as Delta Max and is quantifiedin units, such as millimetres, and is displayed in real time on thescreen. In other words, the values of the two plots for the same degreeof flexion are compared and then the largest difference is taken as theDelta Max value and is displayed.

Note that for all these tests, the icons 242, 244 are highlightedindicating that the reference bodies 100, 110 are in the field of vision256.

One advantage of the present system is that the results on the tests arequantified and displayed in real time to the physician. Since theposition measurement system 20 monitors in real time the positions ofthe two bones due to the reference bodies 100, 110 being attachedthereto, one or more values can be generated that quantify the testresults. As shown in FIG. 13, the results of the four tests can bedisplayed on a single screen. The results have associated values thatare displayed to assist the physician in evaluating the particularpatient's condition and assist in selecting appropriate remedial action,such as reconstructive surgery. For example, the physician can reviewthe results and more specifically, the quantitative values generated anddisplayed for each test and then decide what type of surgical procedureshould be undertaken. More particularly, there is a strong link betweenthe laxity values and a determination of whether extra articularstabilization is needed for the patient. Extra articular stabilizationis a technique by which not only is the anterior ligament (ACL) replacedbut also the ligament is routed to the exterior portion of the knee toprovide extra articular stabilization by being fixed between the tibia 4and the femur 2.

By easily calculating and displaying in real time, the results of thelaxities tests, the present invention provides the physician with a toolthat is used to determine not only how to optimize the placement of theACL but also whether additional surgical procedures may be needed, suchas extra articular stabilization or medial or lateral ligamentreconstruction. The system 10 is preferably a stand-alone unit that isprovided at the operative site and gives the physician a helpful, realtime visual guide.

After performing the above-described pre-operative tests, the next stepis to determine and locate both a tibia point (T₁) and a femur point(F₁) that produces optimal isometry. As is known in the relevant art,isometry involves a measurement of length change between selectedreplacement ligament (e.g., cruciate ligament) insertion site with apassive range of motion. When no length change occurs through a normalunrestricted motion arc, the reconstruction is said to be isometric.According to some conventional thought, anisometry of no more than 2millimetres, through a range of from 0 to 90 degrees, is thought toproduce satisfactory cruciate placement precision. It is thereforedesirable to maximize isometry, thereby minimizing the length change ofthe ligament. As will be described below, while conventional computerassisted surgery system minimized isometry based on a linear modelbetween a point on the tibia 4 and a point on the femur 2, the presentinvention takes into account that in reality, the replacement ligamentis unlikely to be able to travel in a straight line (linear) since boneshapes and in particular the topographical features of the bonesinterfere with the placement and travel of the ligament. Thus, anyimpingement needs to be taken into account when selecting the points T₁and F₁. As set forth below, the present invention is configured tocalculate an isometry value that is truer or closer to the realities ofthe operative site since the topographical features of the bones can betaken into account due to the bone morphing capability of the presentinvention.

Next, an anisometry investigation is undertaken to determine theanisometry characteristics of a selected tibial point and a femoralpoint. As shown in FIG. 14, a tibia point is selected as a well as afemoral point using either the pointer 120 or the drill guide 78 ofdrill 70 (note icons 240, 242, 244 are highlighted). To record eachpoint on the femur 2 and the tibia 4 after the pointer 120 (or drillguide) is positioned on the bone itself as by screwing into the bonesand then the physician activates the actuator (e.g., depressing a footpedal) to record each of these points. As shown in FIG. 14, all threeicons 240, 242, 244 are highlighted since all are in the field ofvision.

One helpful feature of the present invention is shown in FIG. 14, wherethe previously recorded (digitized) points are highlighted and theprecise relationship between the two bones 2, 4 is shown by projectingthe condyles 13, 15 of the femur 2 on to the tibial surface 2. In FIG.14, the projected condyles 13, 15 are shown as shadows on the tibialsurface. The physician can thus see the relationship between thedigitized points of the tibial surface and the projected condyles 13,15. FIG. 15 shows that when investigating and calculating the anisometryof a ligament between the two selected points, it is helpful to plot ina graph 300 (also can be referred to as an anisometry map) the length ofthe ligament on the y axis and on the x axis, the flexion (in degrees)is plotted. In yet another feature of the present invention, the graphincludes indicators, generally indicated at 290, 292 for quicklyindicating to the physician if the system is operating normally. Inother words, the indicator 290 is in the form of an arrow having anincreasing slope, while the indicator 292 is in the form of an arrowhaving a decreasing slope. The arrow 290 is labelled with the word“INCORRECT” to indicate that if the slope of the plotted points (graph)is in the same direction as the arrow 290 (i.e., positive slope), thereis something wrong with the operation since the plotted points shouldinstead have a negative slope of flexion similar to the arrow 292 whichis labelled as “CORRECT”. The arrow 290 can have a red color to indicatean undesirable event since red is associated with a traffic stop ordanger, while the arrow 292 has a green color to indicate properoperation and a desirable event since green is associated with go at atraffic stop, etc.

The drilling device 70 is navigated by aiming the drill tip 74 at thetarget locations on both the tibia 4 and the femur 2. As shown in FIG.15, both the tibia 4 and the femur 2 are displayed on a single screenwith the condyles 13, 15 being superimposed on the tibia 4 in the windowthat shows the tibia 4. It will be appreciated that each window shows animage of the tibia 4 and the femur 2 that is generated using bonemorphing technology as previously mentioned. Bone morphingadvantageously permits the target locations of the tibia 4 and the femur2 to be shown topographically. For example and as shown, the femoralnotch of the femur 2, which was the area of the highest amount ofdigitized points, includes a topographical layout to assist thephysician in selecting a desired location to locate an anchor point inthe femur 2. As shown in the graph 300, a plot is generated toillustrate the anisometry characteristics of a ligament placed betweenthe two selected points T₁ and F₁ and more particular, the graph 300plots of the length of the ligament over a degree of flexion for aligament that is attached between the two selected points. A quickglance of the graph 300 will indicate to the physician that the systemis operating normally and that the physician has correctly performed allsteps since the plotted line has for the most part a negative slope.

As mentioned, the present invention is able to provide the physician inreal time with a quantitative anisometry value to permit the physicianto easily compare different sets of target anchor points in order tominimize the isometry. In FIG. 39, the anisometry value is indicated inor near the graph 300 and in the illustrated embodiment, this value is 4mm. Also of interest, is that region 310 of the screen lists theselected diameter of the replacement ligament and buttons 312, 314 canbe used to either increase or decrease, respectively, the diameter. Thediameter of the replacement ligament is taken into consideration whendetermining the true anisometry value according to an algorithm that isused in the present system 10. As the diameter of the ligamentincreases, the change in trajectory of the ligament creates a biggerimpact on the anisometry value.

As mentioned above, one of the disadvantages of the conventionalcomputer assisted systems for determining the position of the femurpoint and tibia point for minimizing the anisometry is that thesesystems do not account for interference of impingement of the ligamentby the bone surfaces; but rather, they merely calculate anisometry onthe assumption that the ligament follows a straight line between the twopoints. In direct contrast and according to a preferred embodiment ofthe present invention, the locations of the preferred fixation points(anchor points) of the internal and external ligaments on the femur 2and tibia 4 can be computed using an algorithm that detects and accountsfor interference or impingement of the ligament with the surfaces of thebones 2, 4 that have been modeled in a computer-based system accordingto one embodiment of the invention, thereby overcoming the deficienciesassociated with the conventional systems. This is preferablyaccomplished in a computer simulation that uses a geometrical algorithmto simulate the ligament wrapping around or contact any bone modeledsurface that might be along its path between the two selected fixationpoints.

One possible method of computing the ligament course (path of travel) inreal time is to use a numerical simulation that approximates thetrajectory of the ligament. The application of the algorithm can beillustrated with reference to FIG. 28 which illustrates a selectedfixation point T₁ on the tibia 4 and a selected fixation point F₁ on thefemur 2. Next, the fixation point T₁ is connected to the fixation pointF₁ with a “virtual” (initially straight) ligament, shown by a line 320between the two points. A check is then made to see if this virtuallinear ligament 320 extends through or contact any bone surface 2, 4. Inother words, an analysis is performed to see if there is impingement ofthe virtual ligament 320 with one or more of the bone surfaces 2, 4. Ifthere is impingement, then the virtual ligament 320 is divided into anumber of smaller ligament segments with end points R as shown in FIG.29. Each segment with end point R that extends or lies at leastpartially inside a bone (i.e., an impinging ligament segment) isprojected onto the bone surface in the direction of the shortest vector322 from the corresponding point to the bone surface. For purpose ofillustration only, FIG. 29 illustrates the situation where the virtualligament 320 passes through the inside of the femur 2 and thus isimpinged by the femur 2.

Next, the projected ligament is further subdivided and the above stepsare repeated until the virtual ligament 320 no longer substantiallyinterferes with the bone (femur 2). In other words, the ligament issubdivided and the each subdivided ligament segment that still fallswithin the bone (still impinges) is projected onto the bone surface.After the aforementioned has been accomplished, the line representingthe ligament is shifted in a direction normal to the surface by a valueequal to the ligament radius r and if a straight line can be drawnbetween any non-neighboring endpoints within the ligament withoutcrossing the bone, the intermediate endpoints between the neighbors aredeleted (i.e., this simulates ligament tensioning). According to oneembodiment, one of the non-neighboring endpoints is the endpoint R atthe anchor point of one of the bones (in other words, one actual end ofthe ligament), as shown in FIG. 30 by the legends EP₁ (endpoint 1 nearthe femur surface) and EN₈ (endpoint 8 at the anchor point of tibia).The endpoints EN₂ through EN₇ represent other endpoints of the ligamentsegments. As shown in FIG. 68, a straight line can be drawn between EN₄and EN₈ without crossing the bone and therefore, endpoints EN₅-EN₇ canbe eliminated and the ligament path is a straight line between EN₄ andEN₈.

Finally, the ligament diameter is taken into account by shifting thecenterline of the ligament away from the bone surface by radius r sincethe ligament is a three-dimensional object. As shown in FIG. 30, thefinal resulting ligament line 330 is not a straight line, as is the caseof the initial straight line 320, but rather the final simulatedligament 330 has a trajectory that accommodates the topographicalfeatures of the bones and lies entirely outside of the bones 2, 4 (i.e.,does not extend through or impinge any of bones 2, 4).

To minimize the anisometric value and locate the optimal fixationpoints, the physician moves the pointer 120 over both surfaces of bones2, 4 in target areas to locate and find the optimal fixation points thatnot only minimize the anisometry value of the ligament but alsominimizes the impingement, if any, of the ligament. Once again, toselect the fixation points, the physician merely depresses the actuator(foot pedal) or performs some similar action to indicate to the systemthat the location where the pointer tip 124 is located is to be recordedand stored and processed as one of the selected fixation points. It willalso be appreciated that the algorithm is used to calculate theanisometric value throughout the whole range of extension/flexion,thereby generating a true graph 300 that offers and yields a morecomplete anisometric value that is used by the physician to select theproper fixation points and to select the appropriate ligament length tobe used in the operation. Thus, for any two fixation points (T₁ and F₁)that are selected a graph or plot 300 is generated to show the truelength (as determined by the above algorithm) of the ligament over arange of flexion degrees. FIG. 15 shows different fixation points beingselected for analysis of the anisometric data. The present system 10 isthus a real time system that looks and takes into account the anatomy ofthe patient when assessing and determining the optimal fixation pointsfor the ligament.

In yet another aspect, the present invention is configured to assist thephysician in locating and selecting the proper locations for the tunnelsthat are formed in the tibia 4 and the femur 2 that terminate at thedesired fixation points on each bone surface. More specifically, oncethe fixation points are located on the bone surface, the nextconsideration is that the location of the tunnels has to be determinedsince the ends of the ligament are anchored at the fixation points tofixation devices, such as screws, that are disposed within therespective tunnels formed in the bones 2, 4. As shown in FIG. 15, theselected fixation points are indicated in the displays by a “bull's eye”type indicator 400, with the center of the bull's eye indicating thecenter of the selected fixation point.

A tibial tunnel is thus formed in the tibia 4 with the tunnel having aninsertion point at one end and the other end being the end thatterminates at the tibia fixation point formed on the tibial surface. Itwill be appreciate that there are any number of different paths(tunnels) that can be formed that enter the tibia 4 at one location andexit the tibia 4 at the tibial fixation point. In other words, theselection of an insertion point will set or determine the location ofthe tunnel since it must terminate or exit at the fixation point andtherefore, varying the insertion point will alter the entry angle of thetunnel as well as the length of the tunnel. FIG. 16 shows a tibialinsertion point being digitized by use of the pointer 120 and moreparticularly, at this point in time the system is set in a mode todetermine the tunnel location and characteristics, the pointer 120 isplaced on the tibial surface to select an insertion point for thetunnel. It will be noted that icons 240, 242 are highlighted. Once thetwo end points are selected, a simulated tunnel is displayed for thephysician to observe and useful tunnel information is supplied. Thephysician can see if the tunnel angle is appropriate and that the tunnelis located and formed in a sufficient density of bone such thatformation of the tunnel will not result in fracturing of the tibia 4. Inaddition, the present invention contains a feature not found in otherconventional systems in that the user interface and software isprogrammed to calculate the depth of the tunnel and provide thephysician with this value on the display (screen). Since the pointer 120and location of the fixation point are points in the three dimensionalcoordinate system, the distance between the two can easily be calculatedusing a conventional data processing system. It is helpful to providethe physician with the tunnel depth since this permits the physician toselect the appropriate sized anchor or fixation device, such as a screw.Screws come in different standard lengths and therefore, it is desirablefor the physician to try to best match the length of the screw with thedepth of the tunnel so that the screw does not protrude beyond thetibial surface and thereby perhaps interfere with the femur, etc., andequally, it is desirable for the screw to not be too far away from thetibial surface when placed in the tunnel.

In yet another feature, not only is the exit port of the tunneldisplayed (it being desirable for the exit port of the tunnel to matchthe center of the bull's eye 400) but also the periphery shape of theexit port is displayed on the tibial surface. The fixation point (bull'seye) is depicted as a perfect circle; while the peripheral shape of theexit port of the tunnel will vary on a number of different parameters,including the specific path of the tunnel through the bone. The degreeof entry of the tunnel into the bone at insertion point will vary theperipheral shape of the exit port since if the insertion point isrelatively shallow and close to the tibial surface with the tunnelextending substantially across the width of the tibia 4, the exit portwill be more oval shaped as opposed to a circle (this situation islikely not desirable since a shallow tunnel can lead to bone instabilityand fracture when drilling the tunnel). Thus, it is typically desirablefor the peripheral shape of the exit port of the tunnel to moreapproximate a circle.

Similarly, the same steps are taken to determine an optimal femoralinsertion point as shown in FIG. 17. As with the tibial tunnel, data isprovided to the physician that corresponds to the depth of the tunneland the location of the tunnel as well as the peripheral shape of theexit port. Next as shown in FIG. 18, it is desirable to perform animpingement study to estimate any impingement/contact between theligament (fiber) and the roof of the notch of the femur 2. Also, theanisometry of the ligament is displayed in graph/plot 300 with theanisometry value being given (in this case 7 mm) as well as a maximumimpingement value (in this case 11 mm). As shown in FIG. 16 and aspreviously mentioned, the condyles 13, 15 are projected on the tibialsurface since this will permit the physician will locate the tunnelexits ports and the ligament such that the ligament does not lie in thisshadowed area that represents the projected condyles 13, 15 since such aposition will result in 100% impingement of the transplant ligament. Inaddition, after the tibial tunnel is formed, the physician has anopportunity to reselect the femur fixation point to obtain a moreoptimal isometric graph. It will be understood that common drillingtechniques include the formation of a pilot hole first before the entiretunnel is drilled and therefore, the formed pilot hole can be checkedagainst the selected fixation point to see if the exit port of the pilothole matches the center of the bulls eye and if it is outside of sometolerance value, the physician can take appropriate remedial action,such as reselection of the femur fixation point as mentioned above.

Once the tibial and femoral tunnels are formed, the precise length ofthe ligament is computed using the above algorithm that takes intoaccount the topographical features of the bones 2, 4. The use of theabove algorithm takes into account the true trajectory of the ligamentas it passes between the two fixation points and during normalextension/flexion. This results because of the incorporation of bonemorphing technology and techniques into the present system resulting ina truer, more accurate anisometry value being generated and displayedquantitatively to the physician.

A final report or final display screen where various information ispreferably provided to the physician to assist the physician inevaluating the success or failure of the procedure. Preferably, afterthe ligament is reconstructed by being anchored within the two tunnels,a series of post-operative tests are conducted in order to test thepatient's response to the surgery and to test the stability of the kneepost surgery (post ligament reconstruction). In particular, the samelaxity tests are conducted again in the same manner described earlier inreference to the pre-operative stage and the associated quantitativevalues for these laxity tests are displayed. Preferably, the screen issubdivided into regions that permit a graphic display and results foreach laxity test to be shown, for example, simultaneously and thus, thephysician can easily view the results of all the tests on a singlescreen. As with the pre-operative stage, the post-operative pivot shifttest results are quantified in a number of different values, includingthe Delta Max value discussed earlier. An exit screen where thepatient's report is provided and an option or command is provided toexit the application and burn the patient's report on a CD or the someother type of storage medium.

As previously mentioned, the present system 10 is also configured toassist the physician in deciding whether to perform an extra-articularligament graft to improve exterior stabilization of the knee. Forexample, the physician will review all of the quantitative resultsobtained from the pre-operative laxity tests, as shown in the displayscreen of FIG. 13, and based upon the specific findings and values forthis particular patient and based upon the physician's prior experiencein interpreting such data, the physician is better guided in making adetermination whether exterior stabilization is needed, thus requiringan extra-articular ligament graft. For example, now that the presentinvention permits a value to be calculated for the pivot shift test, thephysician can evaluate this value and in his/her experience a pivotshift test value that is above a certain threshold may be an excellentindicator that the patient requires or would benefit from exteriorstabilization of the knee.

If the physician decides to perform external stabilization of the knee,then the physician instructs or commands the system of such intent bypressing an icon or button on the user interface of the present system10. For example, after the laxity test data is displayed as shown inFIG. 13, the physician can press a button or icon that is part of thedisplay screen to command the system of such intent, thereby causing thesystem to display a new set of instructive screens to guide thephysician through the process.

More specifically and according to one embodiment, FIGS. 19-21 aredisplays of screens that assist the physician in performing medialcollateral ligament reconstruction, while FIGS. 22-26 are displays ofscreens that assist the physician in performing lateral collateralligament reconstruction. Both medial and lateral reconstruction involvesusing the bone morphing features of the present invention to determineprecise fixation locations that will yield the best results. FIG. 19illustrates the first step of digitizing the femoral surface ofinsertion of the medial collateral ligament using the pointer 120 in thesame manner described hereinbefore. Preferably, a number of acquiredpoints (e.g., minimum 50 as shown in FIG. 19) are collected using thepointer 120 and the data processing system using the location data forthe reference body 100 attached to the femur 2 and the pointer 120.Based on the collected data and bone morphing procedure, a realisticsimulation of the whole digitized area and surrounding areas is created.The areas of higher accuracy can be highlighted in the graphicrepresentation since these areas represent the areas digitized using thepointer 120 (thus specific data has been acquired for each acquiredpoint).

In medial collateral ligament reconstruction, the ligament is preferablyattached to the tibial tunnel entrance (insertion point) which has beendrilled in the tibia 4 and thus, this point is fixed, leaving itnecessary only to locate the optimal point on the femur 2 to fix/anchorthe MCL. FIG. 20 illustrates the digitization of the entrance orinsertion point of the tibial tunnel using the pointer 120. The tibialtunnel in its entirety is shown in FIG. 20. Next and as shown in FIG.21, a femoral point is selected using the pointer 120—in other words,the pointer 120 is used to digitize on the femur 2 the insertion pointof the MCL. The selection of the femoral point with the pointeractivates the anisometry map 300. Preferably, the bone morphing of thefemur 2 provides the physician with a topographical view of the femur 2as illustrated in FIG. 21 and this serves to guide the physician as towhere the location may be to achieve optimal isometry.

FIG. 21 shows the checking of the anisometry of the MCL by evaluatingthe graph/plot 300 and the anisometry value (e.g., 5 mm in FIG. 21example). The physician can redigitize the insertion point of theligament on the femoral surface with the pointer 120 by simply runningthe pointer 120 along the bone surface and initiating the datacollection process. FIG. 21 shows further checking and evaluation of theanisometry of the MCL and more particularly, simulated three-dimensionalimages of the femur 2 and the tibia 4 are shown with the tibialinsertion point being indicated and the selected insertion point (wherethe pointer tip 124 is located) also being shown on the femoral surface.Graph 300 shows the anisometry characteristics of a ligament extendingbetween these two highlighted points. As with the previously describedACL reconstruction, the physician, in real time, simply moves thepointer along the femoral surface and digitizes different points toevaluate the corresponding anisometry data for these points and acomparison of all these acquired points (prospective femoral insertionpoints) is conducted to determine which insertion point yields the bestresults in terms of anisometry values.

It will be appreciated that when the patient requires both MCL and ACLreconstruction, it is possible for a single ligament to be used bypassing the ligament through the tibial tunnel and then looping it backexteriorly to the femoral insertion point to provide the desiredexterior stabilization.

FIGS. 22-26 show how the present system can be used to perform lateralcollateral ligament (LCL) reconstruction. FIG. 22 shows the use of thepointer 120 to widely digitize the femoral surface of insertion of theexternal lateral ligament and thus permit accurate bone morphing to beperformed. FIG. 22 shows various acquired points on the femoral surface,with each acquired point being indicated by a dot or the like.Preferably, a minimum of 50 acquired points are collected, especially inthe target area. The bone morphing results are displayed, with the moreaccurate areas of the femoral surface being differentiated from thesurrounding areas, as by shadowing these areas or depicting these areasin a different color. Next and as shown in FIG. 23, the Gerdy's tubercleis digitized with the pointer 120. This point is used as the tibialinsertion point for the lateral collateral ligament (LCL). FIG. 24depicts the digitizing of the femur insertion point of the LCL with thepointer 120. Once again, the selection of the femoral point with thepointer 120 activates the anisometry map. The physician can selectvarious femoral points until the optimal anisometry fit is achieved andas soon as this occurs, the physician is ready to drill the tunnel orhole to accommodate the LCL.

For both MCL and LCL reconstructions, the drill device 70 is preferablycalibrated so that the tunnel or hole is formed with a high degree ofprecision.

In FIG. 25, the drill guide 78 of drill device 70 is navigated by aimingthe planned points, which again are indicated and highlighted by bull'seyes. It is possible to re-plan these points with the pointer 120 or theguide. In FIG. 26, the anisometry of the LCL is checked and graph 300provides the physician with the anisometry data and value, while thesimulated bone morphs and locations of the respective tunnels and theLCL are shown.

As mentioned earlier, FIG. 27 is one of the final screens of the userinterface that provide the physician with final comparison data toassess the success of the operation as well as various storage optionsfor all of the collected data.

There are a number of advantages provided by the present system 10 andin particular, conventional systems required the laxity tests to beperformed with mechanical devices and the physician would visually andtemporally assess the knee condition, while the present system 10permits the laxity tests to be performed visually and in a simulatedmanner and displayed on a screen easily viewed by the physician in realtime. In addition, the present invention actually places and calculatesa value for the pivot shift test which is of great assistance to thephysician. To provide this type of simulated, visual results for thelaxity tests using the system 10, the present inventors have discoveredthe importance of collecting and capturing lateral/medial glenoid data(while preserving the glenoid structures as opposed to shaving thesestructures as occurs in other procedures) which is used to formulate thelaxity test results; and similarly, the present system acquires andcollects data for the anterior inter-meniscal ligament, which is used toformulate the final value for the pivot shift test due to the anteriorinter-meniscal ligament point being the preferred femoral referencepoint. More particularly, the present inventors have discovered thatexcellent results are yielded when the lateral and medial glenoid pointsare used as reference points on the tibial surfaces and are tracked asthe laxity tests are performed and thus, the glenoid reference pointsare used in calculating one or more laxity values of the knee.

The following is one exemplary surgical protocol; however, it is notlimiting of the present invention and it will be understood that otherprotocols are equally suitable.

-   -   1. Digitise Tibial malleoli    -   2. Record Leg in Extension position with neutral rotation    -   3. Flextion extension of leg with a “forced” posterior drawer to        reduce laxity    -   4. Sagittal plane determined by fitting a plane to motion    -   5. Plane transformed to tibia from 1^(st) measurement    -   6. Digitize summit of external/internal spines of tibia    -   7. Digitize external/internal glenoid of tibia    -   8. Digitize middle of the anterior inter-meniscal ligament of        tibia    -   9. Digitize middle of the arch of the femoral notch    -   10. Check of flexion angle    -   11. Morph tibial surface in the vicinity of the ACL attachment        and check accuracy    -   12. Morph femoral notch surface in the vicinity of the ACL        attachment and check accuracy    -   13. ANTERIOR DRAWER TEST—GUI displays relative position of tibia        plus acquired tibial glenoid points with respect to femur in        real time    -   14. MEDIAL LATERAL STABILITY TEST    -   15. LACHMAN TEST    -   16. PIVOT SHIFT TEST    -   17. Analyse kinematic data and display laxity results (max        distances)    -   18. Navigation

Thus, in one or more embodiments, the present invention can (1) providean accurate system for positioning both intra-articular andextra-articular ligament grafts between at least two articulating bonesof an joint; (2) can provide a method for realistic simulation ofdeformable ligament trajectories based on the three dimensional shape ofthe bones, the graft diameter, and the graft fixation points; (3) canprovide a system for precisely measuring, analysing, and displaying kneejoint laxities in translation and rotation for a set of prescribed kneemotions; (4) can provide information on the shape of the bonesintra-operatively without requiring expensive medical image data; (5)can provide isometric and impingement data for a ligament graftplacement based on a realistic simulation of the trajectory of adeformable or bendable ligament graft; (6) can provide a system forpassively guiding the drilling of bone tunnels to receive ligamentgrafts; (7) can provide data on the drill tunnel length for aidingimplant screw sizing and selection; (8) can provide a system forcomparing and storing preoperative and postoperative joint laxity data;and (9) optional the hip center can be acquired to determine globalorientation of the femur.

For surgical procedures requiring restorative or reconstruction ofligaments in other locations of the body, bone morphology is preferablyutilized to determine optimal parameters to minimize laxity in theresulting joint.

Thus, it will also be appreciated that while the present invention isdiscussed in terms of ligament reconstruction between the femur 2 andthe tibia 4, it is not limited to such application but rather, it can beimplemented in another application where two structures are movablerelative to one another. Thus, one of these structures can be a hip boneand the other can be an adjacent bone.

While exemplary drawings and specific embodiments of the presentinvention have been described and illustrated, it is to be understoodthat the scope of the present invention is not to be limited to theparticular embodiments discussed. Thus, the embodiments shall beregarded as illustrative rather than restrictive, and it should beunderstood that variations may be made in those embodiments by workersskilled in the art without departing from the scope of the presentinvention as set forth in the claims that follow, and equivalentsthereof.

1. A computer assisted orthopedic surgery system for ligament graftreconstruction where bone morphology is utilized to calculate a laxityparameter.
 2. A computer assisted orthopedic surgery system for ligamentgraft reconstruction which is configured to capture two or morereference points on a bone and has a user interface to permit a bonemorphology template to be selected, wherein a laxity parameter of ajoint between two bones is calculated.
 3. A computer assisted orthopedicsurgery system for ligament graft reconstruction comprising: a systemfor obtaining data indicative of a location for ligament graft placementwith respect to at least a first bone and a second bone, the systemincludes: a position determining device that is capable of tracking therelative movements of the first and second bones using reference bodiesthat are attached to the first and second bones; and a pointer that hasa tip for contacting a surface of at least one of the first and secondbones to capture one or more reference points; and a computer that isconfigured to determine and track intraoperative positions of thereference bodies and the pointer and to provide isometric andimpingement data for a ligament graft placement based on a realisticsimulation of a trajectory of a deformable ligament graft.
 4. The systemof claim 3, wherein the isometric data is determined in part by laxitytest data and the reference points used to calculate the laxity testdata include lateral and medial glenoid reference points.
 5. The systemof claim 4, wherein the laxity test data includes data obtained from atleast two pre-operative laxity tests selected from the group consistingof an anterior drawer test, a medio-lateral stability test, a Lachmantest, and a pivot shift test.
 6. The system of claim 5, wherein one ofthe reference points is an anterior inter-meniscal ligament referencepoint that is used as a primary reference point in the pivot shift test.7. The system of claim 5, wherein the computer is configured tocalculate a quantitative value for the pivot shift test that representsa maximum translation of at least one reference point for a set ofprescribed knee motions, the quantitative value being displayed on adisplay associated with the computer.
 8. The system of claim 7, whereinthe at least one reference point is an anterior inter-meniscal ligamentreference point of a femur which comprises the first bone, while thesecond bone is a tibia and the value of maximum translation isdetermined by comparing, at any given flexion angle, a first position ofthe anterior inter-meniscal ligament reference point in a neutralposition prior to performing the pivot shift test and a second positionof the anterior inter-meniscal ligament reference point after performingthe pivot shift test, with a difference between the first and secondpositions representing a translation value, the value of maximumtranslation being the maximum translation value between the first andsecond positions at the same angle.
 9. The system of claim 3, whereinthe computer is configured to display in real time a deformable model ofthe ligament graft superimposed on three dimensional representations ofthe first and second bones.
 10. The system of claim 3, wherein thecomputer is configured to provide on a display in real time threedimensional representations of the first and second bones as well withthe isometric data being presented in an isometric map displayed on thescreen and which includes an anisometric value to assist the physicianin comparing and optimizing the isometric characteristics of theligament.
 11. The system of claim 3, wherein the computer providesisometric and impingement data for a ligament graft placement based on arealistic simulation of a trajectory of a deformable ligament graft topermit selection of optimal fixation points for the ligament graft, thecomputer being configured to determine a drill tunnel length of bonetunnels that terminate at one of the fixation points and are adapted toreceive the ligament graft as well as aiding implant screw sizing andselection.
 12. The system of claim 3, wherein the computer is configuredto compare, store and display preoperative and postoperative jointlaxity data.
 13. The system of claim 3, wherein the system is configuredto precisely measure, analyze, and display knee joint laxities intranslation and rotation for a set of prescribed knee motions.
 14. Thesystem of claim 3, wherein the relative positions of these referencepoints being used in calculating at least one of the isometric andimpingement data which is displayed on a display associated with thecomputer.
 15. The system of claim 3, wherein the computer includes adisplay that shows in real time images of the first and second bonesgenerated by bone morphing techniques and relative positionstherebetween and a meter that indicates the relative amount of referencepoints that have been captured, the meter being in the form of a barmeter that increases in size as more reference points are captured. 16.The system of claim 3, wherein the computer calculates at least onelaxity parameter of a joint between the first and second bones toprovide the isometric and impingement data for the ligament graftplacement.
 17. The system of claim 16, wherein the at least one laxityparameter is calculated based on a laxity test selected from the groupconsisting of an anterior drawer test, a medial-lateral stability test,a Lachman test, and a pivot shift test, wherein measures values of theat least one laxity parameter is numerically and graphically displayedin real time during the test.
 18. The system of claim 17, furtherincluding a user interface tool which displays the movement of the tibiarelative to the femur during the laxity test, the user interface toolhaving a graphic display to show results of measured values from thelaxity test and graphically illustrate a range of the measured valuesover the range of captured reference points, the user interface toolbeing configured to include a locator that highlights the measured valuefor a current position of the first bone relative to the second bone.19. The system of claim 18, wherein the graphic display is the form ofan elongated indicator showing the range of the measured values, withthe locator being in the form of a ring that surrounds the elongatedindicator and has a different contrast compared to the elongatedindicator, wherein a relative position of the ring axially along theelongated indicator represents how the current measured value comparesto the other measured values.
 20. The system of claim 19, wherein theelongated indicator has along its longitudinal axis, a first portion anda second portion visually identifiable from the first portion with aneutral position of the measured values being indicated where the firstand second portions abut, with the ring being located along thelongitudinal axis to permit easy comparison between the measured valueof the current position of the bones and the neutral position.
 21. Thesystem of claim 3, wherein the computer is configured so that wheninvestigating and calculating the anisometry of the ligament graftreplacement between a first point selected on the first bone and asecond point selected on the second point, an anisometry map isgenerated and visually displayed, a length of the ligament graftreplacement being plotted on the y axis and the flexion in degrees beingplotted on the x axis.
 22. The system of claim 21, wherein theanisometry map includes indicators for quickly indicating to thephysician if the system is operating normally, wherein a first indicatoris in the form of a first arrow having an increasing slope, while asecond indicator is in the form of a second arrow having a decreasingslope, the first arrow indicating that if the slope of plotted points onthe map is in the same direction as the first arrow, a system faultexists.
 23. The system of claim 3, wherein the computer is configured tovisually display in real time a first selected point on the first boneand a second selected point on the second bone, the first and secondpoints representing planned points for insertion of the ligament graftreplacement into the respective bones, the visual display including ananisometry map and the impingement data.
 24. The system of claim 3,wherein the one or more captured reference points represent bone surfacepoints that are then supplied to bone morphing software thatextrapolates these points to obtain a complete a bone surfacerepresentation.
 25. A method for conducting at least one of an ACL, MCLor LCL reconstruction or restoration procedure including the step ofcalculating a laxity value of a joint between two bones.
 26. The methodof claim 25, wherein the calculation step includes adjusting thetrajectory of the ligament based on modeled bone morphology.
 27. Themethod of claim 25, wherein the calculation step involves determining avalue for a pivot shift test and displaying the value on a graphicdisplay.
 28. The method of claim 25, further including displaying ananisometric map on a display, the map plotting a length of the ligamentover a range of flexion degrees, with an anisometric value beingcalculated based on the data plotted on the map.